Positive-Displacement Pump, Method for Operating a Positive-Displacement Pump, and Steering System

ABSTRACT

A positive-displacement pump, in particular a vane-type pump for the delivery of a fluid for a consumer of a steering system includes a rotor, a cam ring, a housing-side control plate, and a cover-side control plate that form, in an assembled state, a rotor assembly. The housing-side control plate is arranged adjacent to a housing face surface in an axial direction and the cover-side control plate is arranged adjacent to a cover face surface in the axial direction. The rotor assembly is movable in the axial direction between the housing face surface and the cover face surface.

The invention relates to a positive-displacement pump, in particular a vane pump for delivering a fluid for a consumer of a steering system. The invention furthermore relates to a method for operating the positive-displacement pump, in particular the vane pump for delivering the fluid for the consumer of the steering system. The invention moreover relates to a steering system.

Vane pumps of a known design are generally constructed in such a manner that a rotor rotates in a cam ring. The cam ring is sealed from one side in each case with an end plate forming a housing and with a control plate. The cam ring has a contour running coaxially or non-coaxially with respect to the axis of the rotor, depending on the design, and forms a pump chamber. Slots are arranged in the circumferential surface of the rotor, the slots running substantially radially over the width thereof and in which radially displaceable vanes are guided. Upon rotation of the rotor about its axis, the vanes are guided along the contour of the cam ring, wherein chambers having a varying volume are in each case formed between two adjacent vanes. In accordance with the rotational movement of the rotor, a suction region and a pump pressure chamber or pressure region are formed, wherein the suction region is arranged in the region of increasing volume, and the pressure region of the pump pressure chamber is arranged in the region of reducing volume, of the chambers.

Conventional vane pumps deliver hydraulic fluid in only one direction of rotation of the vane pump. Furthermore, a control valve has to be provided for regulating a delivering rate of the vane pump.

The invention is therefore based on the object of indicating a positive-displacement pump, in particular a vane pump for delivering a fluid for a consumer of a steering system, which positive-displacement pump is reversible in its direction of rotation and in the case of which a control valve does not have to be provided for regulating a delivery volume of the vane pump.

The object is achieved by a positive-displacement pump, in particular a vane pump for delivering a fluid for a consumer of a steering system having the features of patent claim 1. Furthermore, the object is achieved with a method for operating a positive-displacement pump, in particular a vane pump for delivering a fluid for a consumer of a steering system having the features of patent claim 11. Moreover, the object is achieved with a steering system having the features of patent claim 13.

DISCLOSURE OF THE INVENTION

The present invention provides a positive-displacement pump, in particular a vane pump for delivering a fluid for a consumer of a steering system, wherein a rotor, a cam ring, a housing-side control plate and a cover-side control plate in the assembled state form a rotor assembly, and wherein the housing-side control plate is arranged in the axial direction adjacent to a housing end surface and the cover-side control plate is arranged in the axial direction adjacent to a cover end surface, wherein the rotor assembly is movable in the axial direction between the housing end surface and the cover end surface.

The present invention furthermore provides a method for operating a positive-displacement pump, in particular a vane pump for delivering a fluid for a consumer of a steering system. The method comprises providing a rotor assembly consisting of at least one rotor, a cam ring, a housing-side control plate, and a cover-side control plate which in the assembled state form a rotor assembly, and wherein the housing-side control plate is arranged in the axial direction adjacent to a housing end surface and the cover-side control plate is arranged in the axial direction adjacent to a cover end surface, wherein the rotor assembly is movable in the axial direction between the housing end surface and the cover end surface.

The present invention moreover provides a steering system comprising at least one consumer, and a positive-displacement pump for delivering a fluid for the at least one consumer of the steering system.

One concept of the present invention is to provide a vane pump which delivers in both directions of rotation. Driven by an electric motor which drives in both directions of rotation in accordance with a steering movement, the vane pump delivers hydraulic fluid into corresponding steering cylinder chambers of a steering system. Depending on the direction of rotation, the ports are acted upon in an alternating manner with suction fluid or pressure fluid. Owing to the movability of the rotor assembly in the axial direction between the housing end surface and the cover end surface, separate shuttle valves for supplying pressure behind the vanes and the suction and pressure regulation of the vane pump can be omitted by the rotor assembly being positioned against the housing end surface or the cover end surface.

Advantageous embodiments and developments emerge from the dependent claims and from the description with reference to the figures.

According to a preferred development, it is provided that a direction of rotation of the rotor is reversible, wherein the fluid can be delivered in a first direction of rotation of the rotor or in a second direction of rotation of the rotor, wherein, during operation, a respective pressure side of the positive-displacement pump positions the rotor assembly against a housing end surface or cover end surface lying opposite the pressure side of the positive-displacement pump. It is therefore ensured that the respective pressure side is open at its side behind the vanes and the resulting connection to the suction openings is closed.

According to a further preferred development, it is provided that a channel system, which is formed in the rotor, of a pressure supply behind the vanes is closable in a sealing manner on a side of the rotor assembly bearing against the housing end surface or the cover end surface. On the basis thereof, since the respective pressure side is open at its side behind the vanes, and the resulting connection to the suction openings is closed, separate shuttle valves for supply behind the vanes and the suction and pressure regulation can be omitted.

According to a further preferred development, it is provided that in an inoperative position, the rotor assembly is spaced apart from the housing end surface and the cover end surface, wherein a gap formed between the rotor assembly and in each case the housing end surface and the cover end surface is at least 0.2 mm, preferably at least 0.1 mm. The rotor assembly is therefore slightly lifted off from the two end surfaces in the inoperative position so that, depending on the direction of rotation and therefore depending on the pressure side, the pressure fluid can pass in this intermediate space to the appropriate channels behind the vanes, and the vane pump can easily deliver with immediate support behind the vanes. The dimensions of the gap are dimensioned in an advantageous manner such that only minimum axial movements of the rotor assembly take place.

According to a further preferred development, it is provided that a first end side of the rotor assembly and a second end side of the rotor assembly each have an annular groove into which an O ring formed from an elastic material, or a spring element is fitted, wherein, in order to space the rotor assembly from the housing end surface and the cover end surface, a thickness of the O ring and of the spring element are designed to be greater than a depth of the groove, and wherein a spring constant of the O ring and of the spring element is at least 200 N/mm, preferably at least 300 N/mm. The inoperative position of the rotor assembly is therefore ensured by the O ring seal and/or the spring element.

According to a further preferred development, it is provided that a respective pressure side and suction side of the positive-displacement pump depending on the direction of rotation of the rotor are separated from each other by a sealing ring arranged on an outer side of the cam ring. Effective fluidic separation of the pressure side and suction side of the positive-displacement pump can therefore be achieved in an advantageous manner.

According to a further preferred development, it is provided that the rotor has, on a first end surface or on a second end surface, an annular groove, into which a spring element, preferably a spider spring is inserted, which spring element engages under the inner vane ends of the vanes and positions the vanes against the inner wall of the cam ring. The supply of pressure behind the vanes can therefore be supported from a standing at low rotational speeds of the vane pump.

According to a further preferred development, it is provided that the rotor has, on a first end surface or on a second end surface, an annular groove, into which a tubular body, which is inserted into the housing-side control plate or into the cover-side control plate, projects with an equidistant sliding curve with respect to the cam ring, wherein, when the rotor is started up, the vanes, by contact with the tubular body, are extendable in the radial direction of the rotor in the respective slots for positioning the outer ends of the vanes against the inner wall of the cam ring. The vanes are therefore inevitably extended during starting up, and therefore the vane tips lie in the boundary layer region of the cam. The beginning of the delivery is therefore ensured.

According to a further preferred development, it is provided that the tubular body projects at least 3 mm, preferably at least 2 mm into the groove formed on the first end surface or on the second end surface of the rotor, wherein the tubular body is in the form of a metal sheet and is secured at an annular groove formed in the housing-side control plate or in the cover-side control plate. The tubular body can therefore make contact with the vanes in the starting-up phase of the vane pump in order to extend the vanes.

According to a further preferred development, it is provided that a pressure against the vanes from behind the vanes is controllable by a size difference between an end side of the tubular body, which end side is adjacent to the rotor, and a depth of the annular groove formed in the first end surface or in the second end surface of the rotor, since varying vane cells which are sealed off according to gap size are provided on the rear side of the vane cells.

The described refinements and developments can be combined with one another as desired.

Further possible refinements, developments and implementations of the invention also comprise combinations, not mentioned specifically, of features of the invention described previously or below with respect to the exemplary embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The attached drawings are intended to impart further comprehension of the embodiments of the invention. They illustrate embodiments and serve in conjunction with the description for explaining principles and concepts of the invention.

Other embodiments and many of the advantages mentioned emerge with regard to the drawings. The elements illustrated in the drawings are not necessarily shown true to scale with respect to one another.

In the drawings:

FIG. 1 shows a longitudinal sectional view of a positive-displacement pump for delivering a fluid for a consumer of a steering system according to a preferred embodiment of the invention;

FIG. 2 shows an enlarged detailed view of the positive-displacement pump, which is illustrated in FIG. 1, according to the preferred embodiment of the invention;

FIG. 3 shows a schematic illustration of the positive-displacement pump for delivering the fluid for the consumer of the steering system according to the preferred embodiment of the invention;

FIG. 4 shows a cross-sectional view of the positive-displacement pump for delivering the fluid for the consumer of the steering system according to the preferred embodiment of the invention;

FIG. 5 shows a sequence diagram of a method for operating the positive-displacement pump for delivering the fluid for the consumer of the steering system according to the preferred embodiment of the invention; and

FIG. 6 shows a steering system comprising the positive-displacement pump for delivering the fluid for the consumer of the steering system according to the preferred embodiment of the invention.

In the figures of the drawings, the same reference signs refer to identical or functionally identical elements or components, unless stated otherwise.

FIG. 1 shows a longitudinal sectional view of a positive-displacement pump for delivering a fluid for a consumer of a steering system according to a preferred embodiment of the invention.

The positive-displacement pump 10 is preferably driven by an electric motor 14. The electric motor 14 is connected in an advantageous manner to a rotor 24 by means of a shaft 16. The positive-displacement pump 10 preferably has a pump housing 18 in which a cam ring 20 and the rotor 24, which is mounted rotatably in the latter, are arranged. Furthermore, a housing-side control plate 32 and a cover-side control plate 34 are preferably arranged in the pump housing 18.

The pump housing 18 preferably has a first housing part 18 a and a second housing part 18 b. The first housing part 18 a is preferably arranged adjacent to the electric motor 14 and the shaft 16 and at least partially surrounds the cam ring 20, the housing-side control plate 32 and the cover-side control plate 34. The second housing part 18 b is preferably formed by a cover which closes the first housing part 18 a.

A working chamber 30 is formed between the cam ring 20 and the rotor 24. The working chamber 30 is bounded in the axial direction by an end side 32 a of the housing-side control plate 32 and by an end side 34 a of the cover-side control plate 34. The rotor 24, the cam ring 20, the housing-side control plate 32 and the cover-side control plate 34 form a rotor assembly 35 in the assembled state. The rotor assembly 35 is preferably screwed down by means of two screws 46 a, 46 b. An external thread of the screws 46 a, 46 b engages here in an internal thread of the rotor assembly 35. Alternatively, the rotor assembly 35 can also be threaded onto a plurality of pins.

The housing-side control plate 32 is arranged in the axial direction adjacent to the housing end surface 36 and the cover-side control plate 34 is arranged in the axial direction adjacent to the cover end surface 38. The rotor assembly 35 is preferably movable in the axial direction between the housing end surface 36 and the cover end surface 38.

A first end side 35 a of the rotor assembly 35 and a second end side 35 b of the rotor assembly 35 each have an annular groove 35 c, 35 d. An O ring 41 formed from an elastic material is preferably fitted into the annular groove 35 c, 35 d. Alternatively, for example, a spring element can be fitted into one of the two annular grooves 35 c, 35 d. A thickness of the O ring 41 and of the spring element is preferably designed to be greater than a depth of the groove. The O ring 41 or the spring element are therefore dimensioned to be larger than the groove. As a result, the O ring 41 and the spring element space the rotor assembly 35 from the housing end surface 36 and the cover end surface 38. A spring constant of the O ring and of the spring element is preferably at least 200 N/mm, in particular preferably at least 300 N/mm.

In the present embodiment, the rotor 24 has an annular groove 24 c on a first end surface 24 a. Alternatively, the annular groove 24 c can be formed, for example, on a second end surface 24 b of the rotor. A tubular body 42 is preferably inserted into the annular groove 24 c, said tubular body projecting out of the groove 24 c in the assembled state of the rotor assembly 35 and, when the rotor 24 is started up (not shown in FIG. 1), extends vanes of the rotor 24 by means of contact in the radial direction of the rotor.

A direction of rotation DR of the rotor 24 is preferably reversible. The fluid can therefore preferably be delivered in a first direction of rotation DR1 of the rotor 24 or in a second direction of rotation DR2 of the rotor 24. During operation, a respective pressure side of the positive-displacement pump 10 therefore positions the rotor assembly 35 against a housing end surface or cover end surface 38 lying opposite the pressure side of the positive-displacement pump 10, depending on the direction of rotation DR of the rotor 24. The positive-displacement pump 10 preferably has a constant geometrical delivery volume. Alternatively, the positive-displacement pump 10 can have, for example, a variable geometrical delivery volume.

FIG. 2 shows an enlarged detailed view of the positive-displacement pump, which is illustrated in FIG. 1, according to the preferred embodiment of the invention.

The tubular body 42 preferably projects 2 mm into the groove 24 c formed on the first end surface 24 a of the rotor. Alternatively, the tubular body 42 can project, for example, 3 mm or more into the groove 24 c formed on the first end surface 24 a of the rotor 24. Alternatively, the groove 24 c can be formed, for example, on the second end surface 24 b of the rotor 24. The tubular body 42 is preferably in the form of a metal sheet. The tubular body 42 is secured in an advantageous manner at an annular groove formed in the housing-side control plate 32. Alternatively, the tubular body 42 can be secured, for example, in an annular groove formed in the cover-side control plate.

A pressure against the vanes (not shown in FIG. 2) of the rotor 24 from behind the vanes is preferably controllable by a size difference between an end side 42 a of the tubular body 42, which end side is adjacent to the rotor 24, and a depth of the annular groove 24 c formed in the first end surface 24 a of the rotor. Alternatively, the tubular body can be arranged, for example, in an annular groove formed in the second end surface 24 b of the rotor.

FIG. 3 shows a schematic illustration of the positive-displacement pump for delivering the fluid for the consumer of the steering system according to the preferred embodiment of the invention.

The channel system, which is formed in the rotor 24, of the pressure supply behind the vanes is preferably closable in a sealing manner on the side of the rotor assembly 35 bearing against the housing end surface 36 or the cover end surface 38, depending on the current direction of rotation of the rotor 24. In the inoperative position, the rotor assembly 35 is spaced from the housing end surface 36 and the cover end surface 38. A gap 39 formed between the rotor assembly 35 and in each case the housing end surface 36 and the cover end surface 38 is at least 0.1 mm. Alternatively, the gap 39 can be, for example, at least 0.2 mm.

In an advantageous manner, a respective pressure side DS and suction side SS of the positive-displacement pump 10 depending on the direction of rotation DR of the rotor 24 are separated from each other by a sealing ring 44 arranged on an outer side 20 b of the cam ring. Alternatively, for example, a piston sliding ring can be provided instead of the sealing ring.

FIG. 4 shows a cross-sectional view of the positive-displacement pump for delivering the fluid for the consumer of the steering system according to the preferred embodiment of the invention.

The rotor 24 advantageously has, on the first end surface 24 a, the annular groove 24 c. A spring element 43 in the form of a spider spring is preferably inserted into said annular groove. The spring element 43 engages under inner vane ends 27 b of the vanes 27 and therefore positions the vanes 27 against the inner wall 20 a of the cam ring 20.

As an alternative to the provision of the spring element 43 in the form of the spider spring, it is possible, for example, for the tubular body 42 to be inserted into the annular groove 37 formed in the cover-side control plate 34. The tubular body 42 preferably projects out of the annular groove 37 and into the annular groove 24 c formed on the first end surface 24 a of the rotor 24. The tubular body (not shown in FIG. 4) has an equidistant sliding curve with respect to the cam ring 20, wherein, when the rotor 24 is started up, the vanes 27, by contact with the tubular body, are extendable in the radial direction of the rotor 24 in the respective slots 26 for positioning the outer vane ends 27 a against the inner wall 20 a of the cam ring 20.

FIG. 5 shows a sequence diagram of a method for operating the positive-displacement pump for delivering the fluid for the consumer of the steering system according to the preferred embodiment of the invention.

The method comprises providing S1 a pump housing, a cam ring arranged in the latter and a rotor mounted rotatably in the latter, wherein slots in which vanes are guided in a radially displaceable manner are formed in the rotor. The method furthermore comprises acting S2 upon the vanes with a fluid pressure by means of a channel behind the vanes, which is formed in the rotor and connects the slots to a pressure side of the positive-displacement pump.

Moreover, the method comprises positioning S3 outer ends of the vanes against an inner wall of the cam ring. The method furthermore comprises providing S4 a working chamber which is formed by the cam ring and the rotor and is bounded in the axial direction by an end side of a housing-side control plate and by an end side of a cover-side control plate.

The method furthermore comprises providing S5 a rotor assembly consisting of at least the rotor, the cam ring, the housing-side control plate and the cover-side control plate which in the assembled state form the rotor assembly, and wherein the housing-side control plate is arranged in the axial direction adjacent to a housing end surface and the cover-side control plate is arranged in the axial direction adjacent to a cover end surface, wherein the rotor assembly is movable in the axial direction between the housing end surface and the cover end surface S6.

Furthermore, it is preferably provided that a direction of rotation (DR) of the rotor is reversible, wherein the fluid is delivered in a first direction of rotation (DR1) of the rotor or in a second direction of rotation (DR2) of the rotor, wherein, during operation, a respective pressure side (DS) of the positive-displacement pump positions the rotor assembly against a housing end surface or cover end surface lying opposite the pressure side (DS) of the positive-displacement pump.

FIG. 6 shows a steering system comprising the positive-displacement pump for delivering the fluid for the consumer of the steering system according to the preferred embodiment of the invention.

The steering system 1 is located in a motor-driven vehicle, in particular in a passenger vehicle or a utility vehicle. The steering system 1 comprises a steering wheel 2 which is to be operated by the driver and, via a steering column 3, acts upon a vehicle wheel by means of a steering gear 4 and adjusts said vehicle wheel in accordance with the drivers requirements.

A steering angle and torque sensor 5 is symbolically incorporated into the steering column 3, via which sensor the steering angle, and optionally also the steering angle speed, can expediently be determined and can be supplied as an input signal to a regulating and control unit in which the input signals are processed and which generates therefrom—together with further vehicle state and operating variables—adjustment signals via which the diverse units of the vehicle, including the steering system 1 and the electric motor 9 a, can be adjusted.

The steering system 1 has at least one consumer or adjustment cylinder 6. The adjustment cylinder 6 has two separate chambers 6 a and 6 b which are each connected to the positive-displacement pump 10 via hydraulic lines 8 a, 8 b. The positive-displacement pump 10 is preferably designed as a vane pump. The positive-displacement pump 10 preferably generates a hydraulic pressure for the consumer. Furthermore, an electric motor 9 a is provided which is connected to the positive-displacement pump 10 by means of a coupling 9 b.

Although the present invention has been described above with reference to preferred exemplary embodiments, it is not restricted thereto, but rather can be modified in diverse ways. In particular, the invention can be altered or modified in manifold ways without departing from the essence of the invention.

For example, the tubular body 42 can be secured by a cast groove shape in the annular groove formed in the housing-side control plate 32 or in the cover-side control plate 34. The rotor is preferably UU-shaped or HH-shaped, depending on whether a guide is attached on one side or both sides. Leakage fluid removal preferably takes place via a bearing lubricating groove and shaft sealing ring lubrication to a tank port of the vane pump. Alternatively, the vane pump can be used, for example, as a recuperating pump, for example for generator driving of the electric motor 9 b with a sliding-back steering axle for recovering energy.

LIST OF REFERENCE SIGNS

1 Steering system

2 Steering wheel

3 Steering column

4 Steering gear

5 Steering angle and torque sensor

6 Adjustment cylinder

6 a, 6 b Chamber

8 a, 8 b Hydraulic line

9 a Driving motor

9 b Coupling

10 Positive-displacement pump

14 Electric motor

16 Shaft

18 Pump housing

20 Cam ring

20 a Inner wall of the cam ring

20 b Outer side of the cam ring

24 Rotor

24 a First end surface of the rotor

24 b Second end surface of the rotor

25 Channel system

26 Slots

27 Vane

27 a Outer vane ends

27 b Inner vane ends

28 Channel behind the vanes

30 Working chamber

32 Housing-side control plate

32 a End side of the housing-side control plate

34 Cover-side control plate

34 a End side of the cover-side control plate

35 Rotor assembly

35 a First end side of the rotor assembly

35 b Second end side of the rotor assembly

35 c, 35 d Annular groove of the rotor assembly

36 Housing end surface

37 Annular groove

38 Cover end surface

39 Gap

41 O ring

42 Tubular body

42 a End side of the tubular body

43 Spring element

44 Sealing ring

46 a, 46 b Screws

DS Pressure side

SS Suction side

DR Direction of rotation

DR1 First direction of rotation

DR2 Second direction of rotation

P Fluid pressure 

1. A positive displacement pump for delivering a fluid for a consumer of a steering system, comprising: a pump housing; a cam ring arranged in the pump housing; a rotor mounted rotatably in the cam ring, the rotor having slots in which vanes are guided in a radially displaceable manner, the vanes configured to be acted upon with a fluid pressure by way of a channel located behind the vanes, wherein the channel is formed in the rotor and is configured to connect the slots to a pressure side of the positive displacement pump, and wherein outer vane ends are configured to be positioned against an inner wall of the cam ring; and a working chamber formed by the cam ring and the rotor, the working chamber is bounded in an axial direction by an end side of a housing side control plate and by an end side of a cover side control plate, wherein the rotor, the cam ring the housing side control plate, and the cover side control plate in the assembled state form a rotor assembly, wherein the housing side control plate is arranged in the axial direction adjacent to a housing end surface and the cover side control plate is arranged in the axial direction adjacent to a cover end surface, and wherein the rotor assembly is movable in the axial direction between the housing end surface and the cover end surface.
 2. The positive displacement pump as claimed in claim 1, wherein: a direction of rotation of the rotor is reversible, the fluid can be delivered in a first direction of rotation of the rotor or in a second direction of rotation of the rotor, and during operation, a respective pressure side of the positive displacement pump positions the rotor assembly against the housing end surface or the cover end surface lying opposite the pressure side of the positive displacement pump.
 3. The positive displacement pump as claimed in claim 2, wherein a channel system, which is formed in the rotor, of a pressure supply behind the vanes is closable in a sealing manner on a side of the rotor assembly bearing against the housing end surface or the cover end surface.
 4. The positive displacement pump as claimed in claim 1, wherein: in an inoperative position, the rotor assembly is spaced apart from the housing end surface and the cover end surface, and a gap formed between the rotor assembly and in each case the housing end surface and the cover end surface is at least 0.2 mm.
 5. The positive displacement pump as claimed in claim 1, wherein: a first end side of the rotor assembly and a second end side of the rotor assembly each have an annular groove into which an O ring formed from an elastic material or a spring element is fitted, in order to space the rotor assembly from the housing end surface and the cover end surface, a thickness of the O ring and of the spring element are configured to be greater than a depth of the groove, and a spring constant of the O ring and of the spring element is at least 200 N/mm.
 6. The positive displacement pump as claimed in claim 1, wherein a respective pressure side and suction side of the positive displacement pump depending on the direction of rotation of the rotor are separated from each other by a sealing ring arranged on an outer side of the cam ring.
 7. The positive displacement pump as claimed in claim 1, wherein: the rotor has, on a first end surface or on a second end surface, an annular groove, into which a spring element is inserted, and the spring element engages under the inner vane ends of the vanes and positions the vanes against the inner wall of the cam ring.
 8. The positive displacement pump as claimed in claim 1, wherein: the rotor has, on a first end surface or on a second end surface, an annular groove, into which a tubular body, which is inserted into the housing side control plate or into the cover side control plate, projects with an equidistant sliding curve with respect to the cam ring, and when the rotor is started up, the vanes, by contact with the tubular body, are extendable in the radial direction of the rotor in the respective slots for positioning the outer vane ends against the inner wall of the cam ring.
 9. The positive displacement pump as claimed in claim 8, wherein: the tubular body projects at least 3 mm into the groove formed on the first end surface or on the second end surface of the rotor, and the tubular body is in the form of a metal sheet and is secured at an annular groove formed in the housing side control plate or in the cover side control plate.
 10. The positive displacement pump as claimed in claim 8, wherein a pressure against the vanes from behind the vanes is controllable by a size difference between an end side of the tubular body, which end side is adjacent to the rotor, and a depth of the annular groove formed in the first end surface or in the second end surface of the rotor.
 11. A method for operating a positive displacement pump for delivering a fluid for a consumer of a steering system, comprising: providing a pump housing, a cam ring arranged in the pump housing, and a rotor mounted rotatably in the cam ring, wherein slots in which vanes are guided in a radially displaceable manner are formed in the rotor; acting upon the vanes with a fluid pressure through a channel behind the vanes, which is formed in the rotor and connects the slots to a pressure side of the positive displacement pump; positioning outer ends of the vanes against an inner wall of the cam ring, providing a working chamber which is formed by the cam ring and the rotor and is bounded in an axial direction by an end side of a housing side control plate and by an end side of a cover side control plate; and providing a rotor assembly consisting of at least the rotor, the cam ring, the housing side control plate and the cover side control plate which in the assembled state form the rotor assembly, wherein the housing side control plate is arranged in the axial direction adjacent to a housing end surface and the cover side control plate is arranged in the axial direction adjacent to a cover end surface, and wherein the rotor assembly is movable in the axial direction between the housing end surface and the cover end surface.
 12. The method as claimed in claim 11, wherein: a direction of rotation of the rotor is reversible, the fluid is delivered in a first direction of rotation of the rotor or in a second direction of rotation of the rotor, and during operation, a respective pressure side of the positive displacement pump positions the rotor assembly against a housing end surface or cover end surface lying opposite the pressure side of the positive displacement pump.
 13. A steering system, comprising: at least one consumer; and a positive displacement pump configured to deliver a fluid for the at least one consumer of the steering system, the positive displacement pump comprising a pump housing, a cam ring arranged in the pump housing, a rotor mounted rotatably in the cam ring, the rotor having slots in which vanes are guided in a radially displaceable manner, the vanes configured to be acted upon with a fluid pressure by way of a channel located behind the vanes, wherein the channel is formed in the rotor and is configured to connect the slots to a pressure side of the positive displacement pump, and wherein outer vane ends are configured to be positioned against an inner wall of the cam ring, and a working chamber formed by the cam ring and the rotor, the working chamber bounded in an axial direction by an end side of a housing side control plate and by an end side of a cover side control plate, wherein the rotor, the cam ring, the housing side control plate, and the cover side control plate in the assembled state form a rotor assembly, wherein the housing side control plate is arranged in the axial direction adjacent to a housing end surface and the cover side control plate is arranged in the axial direction adjacent to a cover end surface, and wherein the rotor assembly is movable in the axial direction between the housing end surface and the cover end surface.
 14. The steering system as claimed in claim 13, wherein the positive displacement pump is formed by a vane pump. 